Conventional vehicle steering systems are designed based on a fixed relation between left and right (inner, outer) steer angles to give the best performance at low speeds (close to Ackerman geometry) or can be designed for improved high-speed performance with anti-Ackerman geometry. Due to the fixed relation, the performance cannot be ideal at all speeds. Active steering control has one of the most significant potentials to generate forces to improve handling performance of vehicles at all speeds, where steer angle is actively modified based on driver steering input and vehicle forward speed. Such modification in steer angle with fixed relation between left and right wheels cannot, however, maximize the tire's ability to generate its maximum lateral forces, as it does not consider the tire normal loads which change significantly during maneuvers. In fact, such modification may saturate one tire force while the other tire may still develop more forces.
Analytical studies through computer simulation show that the best handling performance in terms of ideal path, ideal yaw rate, as well as equalization of left and right tires' ability to generate lateral force for a given steering angle at any speed, requires different steering angles for left and right wheels depending on vehicle speed, as well as understeer characteristics of the vehicle. A control algorithm has been developed to show its effectiveness under all conditions. Conventional steering mechanisms use a single degree of freedom (DOF) rack-pinion connected to a 4-bar linkage mechanism for each wheel, following a pro-Ackerman steering ratio between inner and outer wheels. Conventional Active Front Steering (AFS) controls add an active command to this mechanism controlling both front wheels simultaneously.
Ackerman geometry requires the inner wheel to be steered more than the outer wheel, as shown by the following relation:
                                          cot            ⁢                                                  ⁢                          δ              o                                -                      cot            ⁢                                                  ⁢                          δ              i                                      =                              2            ⁢                                                  ⁢                          T              f                                L                                    (        1        )            where δo and δi are the outer and inner wheel steering angles, Tf is the half front track width, and L is the wheel base. This geometric ratio is necessary at low speeds to avoid tire scrub. However, at high speeds, higher steering angle results in higher side slip at the inner tire compared to the outer tire. The lateral force generated by a tire primarily depends on two factors: side slip angle and normal load on the tire. During turning, a load shift from the inner wheels to the outer wheels takes place, which can be significant at high speeds. Low normal load on the inner tire not only reduces its ability to generate lateral force, but also decreases the slip angle at which maximum lateral force can be generated. At high speeds, the inner tire's frictional force is prone to reaching saturation due to low normal load and high-slip angle, while the outer tire may still have the ability to generate more lateral force. Hence, the inner tire becomes a deciding factor for the amount of active steering control.
To overcome this situation, anti-Ackerman geometry is employed in race cars, where, the primary concern is higher lateral forces at high-speed turning. The anti-Ackerman geometry requires outer tires to be steered more, hence generating high lateral force from the more capable tire. However, at the same time, the anti-Ackerman geometry causes tire scrub at low speeds, hence introducing noise and tire wear.
Although such steering concepts can be realized by steer-by-wire and wheel actuator systems, their reliability and safety in vehicle applications is always a major concern.